In recent years, a demand for an optical disc recorder apparatus capable of reproducing and recording information on various kinds of optical discs has been increasing on the market. For example, consumers desire to have an optical disc player and/or recorder that can read and write Recordable/Rewritable Compact Discs (CDs), Recordable/Rewritable Digital Versatile Discs (DVDs), and the like. Typically, such an optical disc player and/or recorder includes at least two kinds of laser diodes such as a CD laser diode and a DVD laser diode.
Optical disc laser power for read channel needs a very fine pitch control so as to make the laser power stability high enough for data read back. Conventionally, the control step size is expected well under 1.0% of controlled target power. In case of 1.0 mW laser emission power for reading or reproducing information, finer than 0.01 mW of power control step size is expected. The current that drives laser diode for generating laser power needs a wide control range to cover both a CD laser diode and a DVD laser diode. The CD laser diode is more sensitive than the DVD laser diode, requiring less driving current than that required by DVD laser diode under the same laser power emission. Thus, for up-to-date optical disc players and/or recorders, the laser diode control circuit should be capable of covering the most sensitive CD laser diode and the least sensitive DVD laser diode, typically ranging from 20 mA to 60 mA.
In the past, the optical disc laser power was controlled by an analog loop. Thus, the control resolution was not a problem, but stability and control ability became problems. Later, the optical disc laser power is controlled by a digital loop. A digital loop is easy to manipulate compared to an analog loop. Conventionally, a single Digital to Analog Converter (DAC) has been utilized in a digital loop to control the laser power for reading. However, this approach causes the contradiction between control resolution and control range, especially when a low cost and low resolution DAC is used. For a single non-idea practical DAC, the wide control range means larger step size; but the larger step size results in rough control resolution. In case the fine control pitch is adapted for higher resolution, the control range will be reduced, resulting in some required control area uncovered.
Therefore, it would be desirable to provide an apparatus of optical disc laser power control which solves problems caused by the contradiction between control resolution and control range, and have both fine control resolution and wide control range features at the same time.